Electrochemical cells play a vital role in today's technology. Major uses include:
batteries for portable power sources PA1 electrowinning of metals including aluminum, copper, zinc and nickel PA1 electrochemical generation of inorganic and organic compounds such as chlorine-caustic and adiponitrile PA1 fuel cells for direct conversion of fuels to electricity PA1 electroplating for decoration and corrosion resistance PA1 separation of the metal from impurities in the leach solution. PA1 electrodeposition from the sulfate. PA1 regeneration of Fe.sup.3+ and/or other oxidizing species at the anode as the depolorizing reaction. PA1 liquid ion exchange reagents for the extraction of cations and anions. PA1 microporous membranes for biological transport, battery separators and other uses.
Many electrochemical cells require the presence of a separator between the positive and negative electrodes. The separator serves to prevent the inter-mixing of anolyte and catholyte and/or gaseous reaction products formed at the electrodes. For example, in the electrowinning of copper from cuprous chloride solution, anodes are enclosed in separator bags to prevent the leakage of cupric (Cu.sup.2+)ions formed in the anodic compartment from reaching the cathodic compartment where they would react with the copper metal deposit, thereby decreasing the current efficiency.
In order for an electrochemical cell to function, one or more ionic species must be transported across the separator to allow for the passage of current. Early separators had no ion specificity and allowed for ionic transport by diffusion of electrolyte fluids. This, of course, resulted in the inter-mixing of anodic and cathodic fluids. Progress in separator technology occurred along two fronts: the development of permselective membranes which discriminated between transport of anions and cations and the development of the microporous separator containing extremely fine pores capable of retarding the flow of larger ions. While both of these approaches provide some advantages, they are far from respresenting the ideal separator. For example, only a few functional groups such as sulfonic acid, carboxylic acid, and alkyl amines have been successfully attached to the polymer structure in a permselective (ion exchange) membrane. Sulfonic and carboxylic acids are cation exchangers with little specificity in discriminating between different cations present in a solution. They have found use in a number of applications where lack of cation discrimination is tolerable. The situation is even less favorable for ion exchangers containing amine functionalities (anion exchangers) since these show instability in many solutions of practical importance and thus have found little commercial use. Additionally, both cation and anion exchange membranes are not impervious to the diffusion of electrolyte and some non-specific transport exists with these membranes.
Microporous separators, as stated above, exhibit discrimination in ion migration as a result of their fine pore size. Again, this discrimination is only qualitative and there remains a significant flow of undesirable ions.
To date, separators for electrochemical processes have been chosen with a compromise in their properties. Low internal resistance requirements suggest that the separator be thin and porous, whereas good separation dictates that the separator be of high selectivity. Overall, the essential properties of a separator for electrochemical applications are (a) adequate chemical stability in the cell environment, (b) high ionic conductivity and (c) low permeability to electrolyte flow.
It is instructive here to discuss several examples of where improved separators for electrochemical cells could lead to significant improvements in technology. Examples are taken from (1) electrowinning, (2) chlor-alkali production and (3) battery separators.